Investigation of the enhancement of N-nitrosomethylbenzylamine-induced esophageal tumorigenesis by 6-phenylhexyl isothiocyanate

Are isothiocyanates potential anti-cancer drugs?

Isothiocyanates are naturally occurring small molecules that are formed from glucosinolate precursors of cruciferous vegetables. Many isothiocyanates, both natural and synthetic, display anticarcinogenic activity because they reduce activation of carcinogens and increase their detoxification. Recent studies show that they exhibit anti-tumor activity by affecting multiple pathways including apoptosis, MAPK signaling, oxidative stress, and cell cycle progression. This review summarizes the current knowledge on isothiocyanates and focuses on their role as potential anti-cancer agents.

Chemoprevention of esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (SCC) is responsible for approximately one-sixth of all cancer-related mortality worldwide. This malignancy has a multifactorial etiology involving several environmental, dietary and genetic factors. Since esophageal cancer has often metastasized at the time of diagnosis, current treatment modalities offer poor survival and cure rates. Chemoprevention offers a viable alternative that could well be effective against the disease. Clinical investigations have shown that primary chemoprevention of this disease is feasible if potent inhibitory agents are identified. The Fischer 344 (F-344) rat model of esophageal SCC has been used extensively to investigate the biology of the disease, and to identify chemopreventive agents that could be useful in human trials. Multiple compounds that inhibit tumor initiation by esophageal carcinogens have been identified using this model. These include several isothiocyanates, diallyl sulfide and polyphenolic compounds. These compounds influence the metabolic activation of esophageal carcinogens resulting in reduced genetic (DNA) damage. Recently, a few agents have been shown to inhibit the progression of preneoplastic lesions in the rat esophagus into tumors. These agents include inhibitors of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), vascular endothelial growth factor (VEGF) and c-Jun [a component of activator protein-1 (AP-1)]. Using a food-based approach to cancer prevention, we have shown that freeze-dried berry preparations inhibit both the initiation and promotion/progression stages of esophageal SCC in F-344 rats. These observations have led to a clinical trial in China to evaluate the ability of freeze-dried strawberries to influence the progression of esophageal dysplasia to SCC.

Suppressive effect of 1,4-phenylene diisothiocyanate on N-butyl-N-(4-hydroxybutyl)nitrosamine-induced urinary bladder carcinogenesis in male ICR mice

The modifying effects of dietary administration of 1,4-phenylene diisothiocyanate (DITC) on N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN)-induced urinary bladder carcinogenesis during the initiation and post-initiation phases were examined in male ICR mice. Five-week-old animals were divided into 5 groups. Groups 1-3 were given BBN (500 ppm) in drinking water for 6 weeks starting at age 6 week. Mice in Group 2 were given the diet containing 100 ppm DITC for 8 weeks during the initiation phase, starting 1 week before BBN exposure. Animals in Group 3 were fed the experimental diet for 24 weeks during the post-initiation phase starting 1 week after the cessation of BBN exposure. Mice in Group 4 were given only the diet containing the test compound, and those in Group 5 were given the basal diet alone throughout the experiment (32 weeks). The frequency of bladder lesions, neoplasms, dysplasia and hyperplasia, was analyzed histopathologically. The cell-proliferation activity estimated by the 5-bromodeoxyuridine labeling index (BrdU-LI), and cell cycle progression by counting cyclin D1-positive cell ratios were compared among the groups using immunohistochemistry. Administration of DITC in the initiation phase reduced significantly the incidence of urinary bladder carcinoma and dysplasia. The frequencies of any lesions of urinary bladder were not reduced by DITC in post-initiation phase. Dietary exposure of this agent in initiation phase reduced significantly both BrdU-LI and cyclin D1-positive cell ratios in any bladder lesions. Administration of DITC in post-initiation phase also significantly reduced BrdU-LI in bladder neoplasms and hyperplasia and cyclin D1-positive cell ratios in urinary bladder carcinoma as well as dysplasia. These results suggest that dietary DITC could be a preventive agent against BBN-induced bladder carcinogenesis in mice when fed during the initiation phase.

Comparison of phenethyl and 6-phenylhexyl isothiocyanate-induced toxicity in rat esophageal cell lines with and without glutathione depletion

Phenethyl isothiocyanate (PEITC) and its synthetic homolog, 6-phenylhexyl isothiocyanate (PHITC), are both potent inhibitors of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung mice tumorigenesis. However, unlike PEITC, PHITC enhanced N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal tumorigenesis. These findings imply that due to its unique chemical properties, PHITC's effects on esophageal cells are procarcinogenic rather than chemopreventive. Relative to PEITC, PHITC is more lipophilic and less reactive, which could result in higher PHITC intracellular levels. Due to ITCs' inherently high level of thiol reactivity, increased intracellular levels of PHITC have the potential to deplete intracellular glutathione (GSH) reserves. Since GSH is a primary intracellular antioxidant and cytoprotective enzyme cofactor, preservation of intracellular GSH status is crucial for cytoprotection. Despite the recognized importance of isothiocyanate structure with the potential for toxicity, no studies have yet investigated the association between the primary intracellular free thiol, GSH, and isothiocyanate-induced toxicity in this target cell population. The present study investigated whether PEITC and PHITC display unique cytotoxic profiles in cultured rat esophageal cells, and also monitored the effects of ITC challenge on cellular GSH status. A final series of experiments investigated the converse i.e., affects of modulation of intracellular GSH status on ITC-mediated toxicity. Dose-response curves revealed that PEITC was significantly more toxic in tumorigenic and non-tumorigenic cells relative to PHITC. The ITC-GSH interaction studies demonstrated comparable GSH levels following either PEITC or PHITC challenge, and also showed that GSH depletion did not augment ITC-mediated cellular toxicity. While our data demonstrate structure related differences in ITC-mediated cytotoxicities, these differences do not appear to be directly attributable to cellular GSH pools.

Etiology and chemoprevention of esophageal squamous cell carcinoma

Squamous cell carcinoma (SCC) of the human esophagus has a multifactorial etiology involving several environmental and/or genetic factors. Current modalities of therapy for this disease offer poor survival and cure rates. Although a number of approaches could be undertaken to reduce the occurrence of esophageal SCC, including changes in lifestyle and improved nutrition, such approaches are not easily implemented. Chemoprevention offers a viable alternative that is likely to be effective against this disease. Clinical investigations in areas of high incidence of esophageal SCC have shown that primary chemoprevention of this disease is feasible, if potent inhibitors are identified. Studies in the Fischer 344 rat model of nitrosamine-induced tumorigenesis have proven valuable in understanding the biology of esophageal SCCs and help identify surrogate end-point biomarkers and putative agents that can be useful in human chemoprevention studies. Several compounds that inhibit tumor initiation by suspected human esophageal carcinogens have been identified using this model. These include diallyl sulfide, isothiocyanates and several polyphenolic compounds. Novel biomarkers, including nuclear/nucleolar morphometry using computer-assisted image analysis of preneoplastic lesions, have been developed to measure efficacy of chemopreventive agents against esophageal SCC. The identification of single agents that inhibit the progression of dysplastic lesions, however, has proven difficult. Results from a food-based approach suggest that the use of freeze-dried berry preparations can affect both initiation and promotion/progression of esophageal SCC in an animal model. These observations provide valuable information for future studies on chemoprevention of cancers of the esophagus in a clinical setting. Given the complex etiology of esophageal SCC, it is felt that the most effective chemoprevention strategies would include agents that reduce mutational events associated with carcinogen exposure in combination with agents that inhibit the progression of intraepithelial dysplasia to invasive cancer.